I totally agree that the words "terrorist" and "terrorism" should not be uttered when describing non-violent acts. To paraphrase brother Joel, it puts "too much noise in the signal." The EFF, in denouncing the digital attacks on MasterCard and others, used the term "cyber-vigilantism" - which I think is a much better term. It reflects that the attacks have real consequences and are motivated by revenge, but are quite distinct from the physical violence that is part and parcel of genuine terrorism.
This sounds right. I was struggling with the language to describe what I saw as a disruptive -- and wrong -- response to the Anonymous hacking spree. 'Cyber-vigilantism' sounds right, and more accurate.